The present invention relates to a technique to write arbitrary refractive index changes in wave-guiding structures without the need for an aperture or a series of apertures. The technique provides unprecedented manufacturing flexibility in research or production environments. More specifically, the present invention may be used to fabricate a variety of waveguide devices that require index of refraction changes along the waveguide length, such as long-period gratings (LPGs) of any length and having any desired refractive index function profile.
Generally to produce photo-induced index perturbations, one shapes the intensity profile of an actinic laser beam by passing the beam through an aperture, series of apertures, or an amplitude mask and then illuminating a photosensitive waveguide with the resulting pattern of actinic radiation. The refractive index is altered in the photosensitive regions of the waveguide that are exposed to this actinic radiation. Periodic devices, such as LPGs, are typically fabricated by passing a laser beam through a periodic amplitude-mask or a series of apertures, but these techniques have several drawbacks. For example, the periodicity of a mask must be changed to fabricate gratings of differing periods, which usually requires that a separate mask be made for each grating period that is desired. In addition, external optics, such as spatial filters, are needed to shape the envelope of the periodic index perturbations which are written into the waveguide for grating apodization, and sub-grating "stitching" is needed to create gratings longer than the amplitude-mask. Typical amplitude masks are about ten centimeters in length. In addition, specialized masks are needed to chirp the refractive index profile, and refractive index modulation functions other than periodic square-waves are difficult to produce. Finally, masks may be damaged by continuous exposure to the high fluences delivered by typical excimer lasers.
Photosensitive waveguides may be exposed to an unmasked UV beam to change the average refractive index of an existing Bragg grating. However, this method merely shifts the wavelength of an existing perturbation pattern and does not allow for the writing of a series of index perturbations, such as those of a long period grating.
Long-period gratings are formed by producing a series of index of refraction perturbations along the length of a waveguide. LPGs are useful temperature and strain sensors. Since LPGs are wavelength-dependent loss elements, they are capable of fine-tuning the spectral characteristics of a device or subsystem to meet certain optical transmission requirements. For instance, inline LPGs may be used to flatten the gain profile of broadband optical amplifiers for wavelength division multiplexing (WDM) systems. In WDM applications, several channels are transmitted simultaneously within the .about.1530- to .about.1560-nm band of an erbium-doped fiber amplifier (EDFA). Each channel is amplified by the EDFA, but the non-uniform gain profile of the EDFA leads to uneven signal amplification between the different channels and hence different signal-to-noise ratios. An LPG device may be fabricated with a loss spectrum that matches the erbium gain spectrum and may be used in an EDFA system to reduce significantly the problem due to uneven signal amplification.
Transmission loss in an LPG can be tailored in various ways, such as by changing the LPGs length, strength, or profile of the refractive index perturbation. To correct the gain characteristics of EDFA systems with LPGs, the amplifier gain spectrum is typically decomposed into a sum of individual constant-period LPG spectral shapes. The appropriate number of filters is fabricated with their shapes and strengths tailored by varying fiber exposure parameters. These gratings are then concatenated to produce a composite transmission spectrum. An LPG-corrected EDFA has been fabricated with a flat gain spectrum over a 40-nm band.
LPGs are made by creating refractive-index perturbations along the fiber with a periodicity much greater than the wavelength of light; in most instances, these periods are on the order of hundreds of microns. The period of the LPG refractive index perturbation is chosen to couple light from a guided mode of a fiber into lossy forward-propagating cladding modes. Coupling from the guided to unguided modes is wavelength-dependent, so spectrally selective loss is obtained.
Researchers have developed point-by-point techniques for LPG fabrication where each index perturbation along the waveguide length is individually written. With these methods, the shape of each index perturbation written in the waveguide is controlled by placing an aperture against the waveguide and irradiating the waveguide through it with actinic radiation. The fiber (or, alternatively, the beam steering assembly) is translated past the writing beam with a precision motion stage, and a mechanical shutter is used to control the radiation dose that is delivered to the fiber in selected locations. This method may eliminate many of the problems encountered with the amplitude-mask fabrication technique, but currently is limited to producing square-wave refractive index profiles. In addition, complicated and exacting motion, variable aperture, and dose delivery control is needed to fabricate chirped and apodized LPGs, reducing the practicality of this technique.
Accordingly, a method is desired for easily and accurately writing index perturbation patterns of any length and having any desired refractive index profile into a waveguide without the use of apertures and/or masks.